I am not an expert, and I know the septic systems are not good, but I can't believe there are so many that a huge body of water like the Gulf would have bad testing based just on the septics. I can see the lakes having a problem more.
But, I think every single septic system here needs to go. Most are outdated and never meant to have modern day usage- multiple showers, bathrooms, washing machines running every day, dishwashers, too many vacationers in the house, etc. I think personal property rights go out the door when they compromise the environment and public health in the community.